That sucks, hope you can fix it, looked like a nice board. What was the carbon/glass layup on this board?

No fix. Goes in the trash.
...
What I would do next time
1. Thicker foam over all, increase to 30mm min.
2. Longer piece of HD foam(?)
3. Deck and bottom would get 4 layers of 250g Uni D carbon about 200mm wide entire length of board

Another option is a couple of wood stringers from the foil mount up to f foot position. Something like 1-2cm wide red cedar or paulownia. Adds a lot of strength only where it's needed and not much of a weight penalty.

I bet you could even do a repair with a few stringers? Put them in from the top glass/carbon patch and cover with deck pad. Seems a shame to bin such a beauty.

Trying to give more strength through changing core materials (be it foam or stringers) is the worst way to do it. Being smart with your layup and lamination materials give you the best results. You don't see F1 Cars, America's Cup Boats, or jet fighters put a denser core into their constructions.

Actually you do.
And. Have you ever seen a structure with carbon skins over honeycomb core with a 3rd "skin" in the middle?
If you haven't, look into AC boats from the past few cycles, and aerospace. Maybe f-1 too....

Was that ply board glassed? Looks like it wasn't. Also, the ply layers look like only a couple very thin hardwood and rest filler. I'm using Baltic birch which has much better strength but am glassing it as well with 2-3x 6 oz. s cloth. Should be bullet-proof.

its 3 layers, plus a glass like hard plastic glued under and around the plate, which was snapped as well...
I have to tell you, holding it in my hand, I was thinking "this is bullet-proof"
It was a high speed crash that broke it...

no.
"3 layers" ply wood. (15mm)
no glassing.
one layer of hard (glass like) plastic under the plate, we put it there to give it an extra strength around the plate. It was glued, and it held up, only to snap together with the board...

Tom, you are right, its five all together...
If you have any idea how to make it stronger,without adding to much extra wheight pls. let me know, the board worked very well, exept for this little mishap.

That's not very high quality plywood. you have 3 layers running longitudinal. The 2 others aren't doing much.
You can get better strength if you go to 5 layer in thinner ply and laminate those together into 15mm or so. If you can get 5mm ply x 3 layers of ply even better.
Then add a 6" or so wide uniD carbon each side ( 2x 250g?) and cover with some 6-7 oz glass.
Cheapest is to get a length of 1m wide Uni D cf and cut it to width.
It won't be lighter but way stronger.

If you can find a piece of 10 mm thick corecell or Divinycell in 80kg/ m3 density and laminate in the rocker using 2 layers to give 20mm thickness, then add CF and glass( more than for ply) it will be lighter.

Over here in Europe you can get a nobile skimboard HF board for 420 UK £, a 2017 Naish 130 HF board for 400, the newer 2018 Naish 130 coming soon has tracks for foil mount, price not posted yet. The Shinn Jackson is 450.